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a solar barn conversion
19 jan 2001
bob wants to convert 864 ft^2 of his half-masonry/half-frame barn
to living space. it looks something like this (in courier font):


                  i           i
             .    i     9'          i   ---
        .         i                 i    4'
    ..............iiiiiiiiiiiii.....i   ---   view from the east
    .                         i     i      
  s .  the kitchen would have i     i    9'   n
    .  a cathedral ceiling--> i     i      

    |             |     |     |     |

            30'     10'   16'   10'      iii is the thermal envelope
    .............iiiiiiiiiiiiiiiiiiii   ---
    .            i      .     .     i
    .            i      .     .     i
s   .         cistern   .     .     i   24'  view from the top
    .            i      .     .     i
    .            i      .     .     i
    .............iiiiiiiiiiiiiiiiiiii   ---

this is a challenge to solar heat. the longest walls face east and west, vs
south, where the sun spends most of the day in wintertime, and there's some
significant shading to the south. the living space has a large surface to
volume ratio, vs a cube. bob wants to use an 8'x8'x12' poured concrete 6,000
gallon cistern under the concrete ground floor for a heat store. it has a 2'
square access hole. can someone spray foam insulation inside without dying
from the fumes? will it stick to the walls even if they are very damp and
covered with green slime? will it need coating with some sort of mastic so
it doesn't break down in hot water over time? maybe glue foamboard to the
inside and install a drop-in epdm or hdpe liner... 

bob plans to raise the north roof 2' and add 2'x24' of south clerestory
windows (w*) and a couple of large east windows (w):

                  2'    w*                  
                 --- .        i
         solar   .                  i   ---
    panels   .    i     9' w    w   i    6'
        .         i living space    i      
    ..............iiiiiiiiiiiii.....i   ---
    .                         i     i      
  s .                         ikitchi    9'   n
    .                         i     i      

    |             |     |     |     |
            30'     10'   16'   10'    

if the clerestory windows have u0.5 thermal conductance and 80% solar
transmission (shgc) and an r15 movable shutter that insulates them
for 18 hours on an average january day, their average conductance is
2'x24'(6hxu0.5+18h/r17)/24h = 8 btu/h-f. if the new east windows are
4'x4', with us u0.25 thermal conductance and 50% solar transmission,
they add 32ft^2xu0.25 = 8 to the living space conductance. 

if the rest of the external surfaces have r20 insulation (5" beadboard
or 4" styrofoam or 6" fiberglass) and the living space is made airtight,
with natural air infiltration of about 0.2 air changes per hour (20x0.2
= 4 ach with a 50 pascal blower door test), the living space to outdoor
conductance is 16 for windows plus 594ft^2/r20 = 30 for kitchen walls
plus about 928ft^2/r20 = 46 for upstairs walls plus 864ft^2/r20 = 43
each for ceiling and floor plus 0.2(864x10)/60 = 29 cfm of air leaks
with a conductance of about 29. the total is 207 btu/h-f.

keeping the living space 70 f on an average 30 f january day requires
24h(70f-30f)207btu/h-f = 199k btu of heat. nrel says 1000 btu/ft^2-day
falls on a south wall and 420 falls on east windows, so the rooms would
gain 0.8x2'x24'x1000 = 38.4k btu from the clerestories, if they had 2'
tall glass. a light-colored south roof reflection might add 20%, making
it 46k. the east windows gather 0.5x32x420 = 6.7k, and 600 kwh/month of
indoor electrical energy consumption could add 68k btu/day. the window
and electrical gains total 120.9k btu/day, so the solar heating system
needs to supply 199k-120.7k = 78k btu on an average day.

an efficient solar water heating panel might gain 800 btu/ft^2-day. with
130 f water inside, it might lose 6h(130-30)1ft^2/r2 = 300, for a net gain
of 500 btu/ft^2. we could solar heat this space with 78k/500 = 156 ft^2 of
panels, something like 4 4'x10' 250 pound used panels from jade mountain
(800) 442-1972/ for $295 each plus shipping from
colorado plus installation ($1,000?), or from solar techniques in phila
(215) 844-4196/ we could tilt them up on the south roof
in 2 east-west rows without shading themselves or the clerestories...

or (this is plan b), bob might double the living space and raise the ridge
8' to admit more sun (with a 4' overhang that's reflective underneath) and
steepen the north roof pitch to 23 degrees and focus some of the sun onto
a 4'x24' horizontal solar water collector shelf inside the windows... 

                  ---    w* 
                   8'    w*   i
                  ---    w*cc   i        cc is the collector shelf
                     i             i
                  i                 i   ---
             .    i      9'  w   w  i    6'
        .         i                 i      
    ..............i.................i   ---
    .             i                 i      
  s .             i                 i    9'   n
    .             i                 i      

    |             |     |           |
          30'       10'      26'     

the 8' clerestories have an average conductance of 32 btu/h-f, and the
east windows add 8. r20 external surfaces and 0.2 ach of air leaks make
the conductance to outdoors 40 plus 43 each for the ceiling and floor plus
2000 ft^2/r20 = 100 for walls plus 0.2(24'x36'x16')/60 = 46 cfm of air
infiltration. the total is 272 btu/h-f.

keeping the living space 70 f requires 24h(70f-30f)272btu/h-f = 261k btu
of heat. the 4' overhang adds about 1.7' to the solar aperture height, so
the clerestories transmit about 0.8x9.7'x24'x1000 = 186k btu/day of sun,
or 223.5k with a light-colored roof. east windows add 6.7k btu. window
gains total 230.2k btu/day, so the electrical energy consumption only
needs to be 261k-230.2k = 30.8k btu/day, ie a frugal 30.8k/3.41kx30
= 271 kwh/month.

that makes the solar energy that enters the living space equal the heat
energy that leaves on an average january day. we also need to provide
hot water (which can heat the rooms with a greywater heat exchanger)
and overnight and cloudy day heat storage. the masonry walls might help
with this if they have insulation on the outside and underground at
the perimeter. 

nrel says 729 btu/ft^2 of beam sun falls on a 1-axis ew concentrator on
an average january day. the clerestories might transmit 0.8x9.7x24x729
= 136k, with a 90% parabolic reflector to focus 122k onto a 4'x24' water
collector below with about 3h(130f-70f)4'x24'/r0.67 = 26k of heat loss to
the rooms, storing 96k btu/day in the cistern, likely enough for showers
and overnight heat storage, with a night setback and some heat stored in
the thermal mass of the living space, eg a hydronic slab. 

the 4'x24' collecting shelf might be a $75 epdm rubber bladder over an
$18 osb shelf over 2x6s with 6" of fiberglass insulation underneath and
more osb under that. the north edge of the shelf might hang from vertical
2x4s to the ceiling every 8'.

three thermal shutters on 8' centers could also be reticulated parabolic
reflectors, using 3" foil-faced foamboard hinged at the top edge, with a
one-way hinge 6' below that (4' above the north edge of the collector)
that won't open past 180 degrees to allow the shutters to fully-open and
lie flat up against the ceiling with one small winch cable attached to
the center of the bottom edge. they could be partially-opened to admit
light to the room and collect hot water, or fully-closed and supported
by stops underneath to insulate the clerestories with their parabolic
shape, lowering heat loss at night or heat gain in the summer.

in an alternative embodiment with fewer moving parts, the reflector could
be 95% metalized mylar film over parabolically-kerfed bows to transmit 5%
of the sun while collecting 95% as heat. this might have no moving parts
if the mylar film(s) provides enough insulation. if not, the clerestory or
mylar cavities might be filled with soap bubble foam at night. or, we might
try filling a clear poly film pillow with bubbles to make a reflector...


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